1. Field of the Invention
The present invention relates to a drive device that performs light-emitting drive of a light-emitting panel wherein capacitative light-emitting elements such as organic electroluminescent elements are arranged in matrix fashion.
2. Description of the Related Art
In recent years, as display devices have become of large size, thin display devices are being demanded and various types of thin display devices are being put into practice. Organic electroluminescent elements (hereinbelow simply referred to as EL elements) are known as one type of display element employed in such thin display devices.
EL elements are capacitative light-emitting elements that may in electrical terms be equivalently represented by a capacitative constituent C and a diode-characteristic constituent coupled in parallel with this capacitative constituent, as shown in FIG. 1. When a DC light-emitting drive voltage is applied between the electrodes of the EL element, electrical charge is accumulated on capacitative constituent C and when the barrier voltage or light-emitting threshold voltage that is characteristic of this element is exceeded current starts to flow from the electrode (anode side of the diode constituent E) to the organic functional layer that performs the role of light-emitting layer, thereby causing this to emit light with an intensity proportion to this current.
FIG. 2 of the accompanying drawings is a view showing the voltage V-current I-brightness L characteristic of such an EL element.
As shown in FIG. 2, the characteristic of an EL element is similar to that of a diode; at voltages below the light-emitting threshold voltage Vth, the current I is very small while at voltages at or above the light-emitting threshold value Vth the current increases abruptly. Also, the brightness L is practically proportional to the current I. That is, if a drive voltage exceeding the light-emitting threshold value voltage Vth is applied, a light-emitting brightness proportional to the current produced by this drive voltage is presented while if the drive voltage is below the light-emitting threshold voltage Vth no drive current flows and the light-emitting brightness remains at zero.
FIG. 3 of the accompanying drawings diagrammatically illustrates the construction of an EL display device in which is mounted a light-emitting panel constituted by a matrix arrangement of such EL elements.
In FIG. 3, n cathode leads (metallic electrodes) B1 to Bn are arranged in parallel in the horizontal direction in light-emitting panel 11 while m anode leads (transparent electrodes) A1, to Am, are arranged in parallel in the vertical direction, respectively, EL elements E1,1 to Em,n being formed at the intersections (total of nxc3x97m intersections). The EL elements E1,1 to Em,n that play the role of pixels are arranged in lattice fashion, corresponding to the positions of intersections of anode leads A1 to Am along the vertical direction and cathode leads B1 to Bn along the horizontal direction, with one terminal thereof being connected to the anode lead (anode lead side of diode constituent E of the above equivalent circuit) and their other terminals (cathode lead side of diode constituent E of the above equivalent circuit) being connected with the cathode leads.
Light emission control circuit 12 respectively controls cathode lead scanning circuit 13 and anode lead driver 14 such that an image representing the video data is caused to be displayed in accordance with this input video data. Specifically, light emission control circuit 12 supplies to cathode lead scanning circuit 13 scanning pulse signal SP such as to make the respective EL elements E1,1 to Em,n capable of being driven, one horizontal scanning line at a time. Furthermore, light emission control circuit 12 generates drive pulses having a logic level corresponding to the input video data and supplies these drive pulses to anode lead driver 14, one horizontal scanning line (GP1 to GPm) at a time. Cathode lead scanning circuit 13 includes scanning switches 51 to 5n corresponding to the cathode leads B1 to Bn that individually determine the voltages of the cathode leads. Scanning switches 51 to 5n respectively apply earth potential (0 V) to the corresponding cathode lead during the period in which scanning pulse signal SP is applied from light emission control circuit 12 and in periods other than this apply bias potential Vcc (for example 10 V) thereto. The bias potential Vcc is applied in order to prevent crosstalk light emission by EL elements respectively connected to respective cathode leads to which scanning pulse signal SP is not supplied and is normally set at bias potential Vcc=VF Anode power source circuit 10 generates a prescribed anode power source voltage VA constituting the source of drive current supplied to respective anode leads A1 to Am in order to drive respective EL elements E1,1 to Em,n in accordance with the power source voltage from battery 100; this is then supplied to anode lead driver 14. Anode lead driver 14 comprises anode drive switches 61 to 6m and constant current drivers 21 to 2m constituting current sources that supply drive current respectively to the EL elements E1,1 to Em,n through anode leads A1 to Am respectively, of light-emitting panel 11. Constant current drivers 21 to 2m respectively generate the above drive currents having a prescribed constant current in accordance with anode power source voltage VA supplied from anode power source circuit 10 and output these respectively to anode drive switches 61 to 6m. Anode drive switches 6 connect the output terminal of constant current drivers 2 to anode leads A if the drive pulse GP supplied from light emission control circuit 12 is for example logic level xe2x80x9c1xe2x80x9d and apply earth potential to the anode leads A if the drive pulse GP is logic level xe2x80x9c0xe2x80x9d. For example, anode drive switch 61 connects the output terminal of constant current driver 21 to anode lead A1 if the drive pulse GP1 supplied from light emission control circuit 12 is for example logic level xe2x80x9c1xe2x80x9d and applies earth potential to the anode lead A1 if the drive pulse GP1 is logic level xe2x80x9c0xe2x80x9d. Also, anode drive switch 6m connects the output terminal of constant current driver 2m to anode lead Am if the drive pulse GPm supplied from light emission control circuit 12 is for example logic level xe2x80x9c1xe2x80x9d and applies earth potential to the anode lead Am if the drive pulse GPm is logic level xe2x80x9c0xe2x80x9d. The amounts of current supplied by the respective constant current drivers 21 to 2m are the current amounts necessary to maintain a condition in which an EL element is emitting light with the desired instantaneous brightness (hereinbelow, this condition is called the xe2x80x9csteady light emission conditionxe2x80x9d). Also, when an EL element is in the steady light emission condition, charge is stored on the capacitative constituent C of this EL element, so the voltage across the two terminals of the EL element is a positive voltage VF somewhat higher than the light-emitting threshold voltage Vth (this voltage is called the forward voltage). Consequently, only the EL elements on the cathode lead that is set to earth potential in response to the scanning pulse signal SP emit light in response to the drive current that is supplied from constant current drivers 2. Of the respective anode drive switches 61 to 6m, only the anode switches that are supplied with drive pulses of logic level xe2x80x9c1xe2x80x9d from light emission control circuit 12 apply drive current on the corresponding anode lead. The respective EL elements E1,1 to Ei,j that are provided in light-emitting panel 11 are thereby made to assume a light emission condition (light-emitting or non-light-emitting) in response to the input video (image) data.
The condition of the light-emitting panel 11 shown in FIG. 3 illustrates by way of example a condition in which cathode lead B1 is driven (scanned) and the EL elements E1,1 and E2,1 connected to this cathode lead B1 are lit. In FIG. 3, EL elements in light-emitting condition are indicated by diode symbols and EL elements in non-light-emitting condition are indicated by capacitor symbols, respectively.
In the condition shown in FIG. 3, cathode electrode lead B1 is driven by only scanning switch 51 being changed over to the earth potential side 0 V. Bias voltage Vcc is applied by scanning switches 52 to 5n to the other cathode leads B2 to Bn. Concurrently, drive current from constant current drivers 21 and 22 is applied to anode leads A1 and A2 by anode drive switches 61 and 62. Consequently, in this case, only EL elements E1,1 and E2,1 are biased in the forward direction, thereby allowing drive current to flow as shown by the arrows from constant current drivers 21 and, so that only EL elements E1,1 and E2,1 are lit.
The relationship between the forward voltage and the drive current applied to the EL elements changes in accordance with the change of temperature as shown in FIG. 4. In addition, as shown in FIG. 5, as is known, this forward voltage rises with lapse of time. In some cases, change of the forward voltage of the EL element with temperature and/or time may make it impossible for constant current drivers 2 to drive the EL element with the prescribed constant current. In this situation, in order to avoid this problem, consideration has been given to supplying anode power source voltage VA on the high side beforehand to constant current drivers 2, but this leads to the problem that if the power source voltage is made high, power consumption is also increased.
Accordingly, in an EL display device as shown in FIG. 3, the above problem is solved by the provision of an anode voltage detection circuit 15 and forward voltage input circuit 16.
Anode voltage detection circuit 15 detects the voltage on one or other of the anode leads A1 to Am (anode lead Am in FIG. 3) and supplies an anode voltage value VD indicating this voltage value to forward voltage input circuit 16. Forward voltage input circuit 16 inputs as the forward voltage value generated on the anode lead the anode voltage value VD supplied from anode voltage detection circuit 15, and supplies forward voltage value VF indicating this to anode power source circuit 10.
Anode power source circuit 10 adjusts the value of the anode power source voltage VA that is to be supplied to the respective constant current drivers 21 to 2m so as to be equal to a voltage value obtained by adding to the above forward voltage value VF the loss voltage generated in constant current drivers 2. In other words, anode power source circuit 10 performs adjustment such as to lower the anode power source voltage VA if the voltage value of the anode power source voltage VA is higher than the voltage value required when the EL elements are maintaining a steady light-emitting condition and such as to raise the anode power source voltage VA if it is lower than this. By such power source voltage adjustment, even if the forward voltage VF changes due to the temperature change or change with time of the EL elements, an anode power source voltage can be generated having an optimum voltage value tracking such changes.
However, in order to input the forward voltage generated on the anode lead, it is necessary that drive current should be flowing on this anode lead. Whether or not drive current is flowing on the anode lead depends on the input video data. Accordingly, whether or not drive current is flowing in the anode lead Am is determined by constantly monitoring whether or not drive pulse GPm is logic level xe2x80x9c1xe2x80x9d by forward voltage input circuit 16 shown in FIG. 3. Thus it is arranged that forward voltage input circuit 16 inputs the anode voltage value VD as forward voltage value VF only if it is determined that drive pulse GPm is logic level xe2x80x9c1xe2x80x9d i.e., if it is determined that drive current is flowing on anode lead Am.
However, with such a construction, the monitoring action as to whether or not drive current is flowing on anode lead Am must be carried out constantly, so there is the problem of considerable wasted power consumption.
In addition, since whether or not drive current is flowing on anode lead Am depends on the input video data, there is the problem that the nature of the display content could produce an absence of opportunities for drive current to flow on anode lead Am, thereby making it impossible to adjust the anode power source voltage VA.
An object of the present invention is to provide a drive device for a light-emitting panel whereby the anode power source voltage can be automatically adjusted to an optimum value with low power consumption and in a reliable fashion.
According to one aspect of the present invention, there is provided a drive device suitable for a light-emitting panel that includes a plurality of mutually intersecting anode leads and cathode leads and a plurality of light-emitting elements connected between said anode leads and said cathode leads at the intersections of said anode leads and said cathode leads, in which said respective light-emitting elements in the light-emitting panel are made to selectively emit light in response to information data. The drive device includes: an anode power source circuit that generates anode power source voltage; a current source that generates drive current to cause said light-emitting elements to emit light using said anode power source voltage; an anode drive switch that supplies said drive current selectively to said respective anode leads in response to said information data; an anode voltage detection circuit that designates a prescribed anode lead of said respective anode leads as an anode lead that is the subject of detection and obtains an anode voltage value by detecting a voltage value on this anode lead designated as the subject of detection; a control circuit that supplies to said anode drive switch prescribed information data to cause said drive current to be supplied in respect of at least said anode lead designated as the subject of detection; and a forward voltage input circuit that inputs as forward voltage value said anode voltage value only while said prescribed information data is being supplied to said anode drive switch; in which said anode power source circuit adjusts said anode power source voltage in response to said forward voltage value that is input by said forward voltage input circuit.
The prescribed anode lead in the respective anode leads of the light-emitting panel is designated as the subject of voltage detection. First, image display is performed based on prescribed image data in regard to which drive current is supplied in respect of at least this anode lead that is designated as the subject of voltage detection. Then, it is arranged that the voltage value on this anode lead that has been thus designated as the subject of detection is input as the forward voltage value, only while this display is being performed, and the anode power source voltage to be applied to the anode lead is adjusted in accordance with this forward voltage value. Consequently, since, while image display in accordance with prescribed image data is being effected as described above, drive current must of necessity be flowing on the anode lead that is the subject of detection, the anode power source voltage can be adjusted to a suitable value in a reliable fashion. Further, since a construction for determining whether or not drive current is flowing on the anode lead that is the subject of detection is unnecessary, the circuit construction can be made of small size, reducing the power consumption.
According to another aspect of the present invention, there is provided a portable terminal device comprising a light-emitting panel. The light-emitting panel includes a plurality of mutually intersecting anode leads and cathode leads and a plurality of light-emitting elements connected between said anode leads and said cathode leads at the intersections of said anode leads and said cathode leads. The portable terminal device comprises: a transmitting/receiving circuit that performs transmission and reception of information data; a battery that generates power source voltage; an anode power source circuit that generates anode power source voltage using said power source voltage; a current source that generates drive current to cause said light-emitting elements to emit light using said anode power source voltage; an anode drive switch that supplies said drive current selectively to said respective anode leads in response to said information data; an anode voltage detection circuit that designates a prescribed anode lead of said respective anode leads as an anode lead that is the subject of detection and obtains an anode voltage value by detecting a voltage value on this anode lead designated as the subject of detection; a control circuit that supplies to said anode drive switch prescribed information data to cause said drive current to be supplied in respect of at least said anode lead designated as the subject of detection; and a forward voltage input circuit that inputs as forward voltage value said anode voltage value only while said prescribed information data is being supplied to said anode drive switch; in which said anode power source circuit adjusts said anode power source voltage in response to said forward voltage value that is input by said forward voltage input circuit.